Electron cryomicroscopy (cryo-EM) is revolutionizing the study of biological macromolecules, because cryo-EM images of individual macromolecular particles can be combined, in processes called single-particle reconstruction and subtomogram averaging, to yield atomic structures without the need for crystallization and in many cases without the need for strict conformational homogeneity. Macromolecular structures, in turn, are essential to the detailed understanding of processes such as the stepping of the kinesin molecular motor or the inhibition of HIV multiplication by tethering molecules in infected cells. Dozens of NIH-funded research projects like these, in more than 16 laboratories at our institution, are being pursued using cryo-EM methods. A severe bottleneck is however the development and screening of cryo-EM specimens. These steps are the most labor-intensive ones of cryo-EM structure projects, but are critical before high-resolution data can be collected. With this application funds are requested to purchase an FEI Talos 200kV electron microscope with autoloader. This is a microscope with a field-emission electron source (essential for high resolution) and a cryogenic specimen autoloader (for high throughput). This microscope will greatly expand the capacity of our cryo-EM facility, and lower the barrier for investigators to employ cryo-EM technology. The microscope will be used for specimen screening and optimization, as well as cryo-EM data collection at sub-nanometer resolution.